This is reminiscent of a girl I had once. It ended up that me and the other dude never really reconciled after it was all out in the open. Truthfully, I felt exactly how you feel right now; I thought I would be the better choice because I wasn't just "out for ass" as my friends say. Either way you're the one who sees the situation for what it really is because obviously neither of them do. So here's what you do: ask yourself whether or not this chick is worthy of fucking up your friendship with your boy. If the answer is "yes", or you really aren't good friends with this guy, then let the girl know how you feel about her, without mentioning your friend. Make it totally about you and her, nothing about anyone else. Bam, there's her ultimatum, either she's on the same page as you, or she's not. If she is, then you're set.